This has happened for so long.. I think it happens when a player kills a neutral monster in fog of war (specially red buff) and then appear somewhere to gank.

What I find hilarious is that if you are (let's say) at their red buff the moment their jungler ganks top (3 min??) and you spot him right next to you on minimap he actually appears in front of you for a brief second just to show up top in the next second. This has happened to me 1 time at least.